Mi Cas A es su Cas A

By Phil Plait | August 29, 2006 7:43 am

In the year 1680, a star in our Galaxy blew up. It wasn’t noticed on Earth, because even though it’s relatively close as these events go — 10,000 light years away — it was behind a thick layer of gas and dust, so its light was dimmed (see the footnote).

But now, 300+ years later, the expanding debris from the explosion is visible. It was first seen as a radio source, and was named Cas A, the first radio source detected in the constellation Cassiopeia. It’s now been observed across the electromagnetic spectrum, and is a pretty cool object. The image displayed here was from Hubble, and was just released today.

We think the star that blew up was about 40 times the mass of the Sun. It lived a violent life, shedding most of its outer layers before it finally exploded. All that sloughed-off gas expanded outward, and when the core exploded the ejecta screamed outward much faster, slamming into the gas, ramming it, making it glow. It’s now a roughly spherical shell of material expanding at tens of millions of kilometers per hour.

As it happens, Cas A was the subject of an educational activity I developed with the help of a team of other folks, both at my home school of Sonoma State University and at Gettysburg College. While developing the activity I calculated some amazing things about the nebula.

The amount of iron in the nebula — just the iron — is over 1000 times the mass of the Earth. That iron was created in the blast, alchemically converted from lighter elements. In fact it’s thought that all the iron in the Universe was made in such explosions. The supernova that made Cas A also created more than enough iron to account for all the iron in a star like the Sun. In the long run, over the lifetime of the the Galaxy, these explosions (which occur roughly once or twice per century) generated enough iron to make 300 million stars like the Sun!

Calcium (as well as a slew of other elements) was created as well. The mass of the calcium in Cas A is very roughly 1029 grams, enough to make 1028 gallons of milk! If you’re curious, there are about 5 grams of calcium in a gallon of milk (I’m rounding the big numbers here pretty severely).

I think that’s all pretty amazing. The iron in your blood and the calcium in your bones were forged in the fires of the death throes of some unknown massive star billions of years ago. The matter was ejected, shot into the Galaxy, where it slammed into and merged with a cloud of gas and dust. That floating junk — the "ash", essentially, from a tremendous explosion — formed the Sun, the planets, and eventually… you.

Footnote: Yes, I know– some of you will think that it didn’t blow up in 1680, that’s just when the light reached us, and it really blew up 10,000 years ago. I disagree. We cannot say anything about that event until the light reaches us, and in a real sense that event has not happened until the light reaches us. Time flows like light, I sometimes say, meaning that the event itself happens when the light reaches us. So it is acceptable to say that the explosion actually happened in 1680.

Image credit: Robert A. Fesen (Dartmouth College, USA) and James
Long (ESA/Hubble), NASA, ESA, and the Hubble Heritage (STScI/AURA)-
ESA/Hubble Collaboration

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA, Science

Comments (28)

  1. Nice post. I’ll have to remember your numbers on iron and calcium when I start teaching my kids about astronomy.

    In regards to your footnote…

    While it may be helpful to some to consider that the event occured 10,000+320 years ago, I agree with your assessment that the part of the universe that is outside of our past light cone is completely unknown to us. What’s more, from a cosmological perspective, it is really meaningless to assign a specific date to an event unless you also specify a reference frame. The perception of an event will vary from observer to observer depending on their positions and velocities relative to the event. Special relativity was developed to address just this kind of observer bias.

    Anyway, keep up the good posting.

    Ad Astra Per Aspera

  2. Cheese Hater

    What’s a “klight year?” :p

    My brother dispises astronomy, he thinks everything astronomer’s believe about the universe is false, and in order to understand it completly we have to think of astronomy like the stock market. Yes I said the stock market!

    I think that the saying “The same stuff as stars” could fit perfectly with this topic.

  3. schwa sticker

    Phil, PhysBrain: I think this point in the footnote really is an important one, as the concept of “when” is really nonsensical if you don’t specify your reference frame.

    Attempting to paraphrase what i HOPE i remember from Brian Greene’s “The Fabric of the Cosmos”: Imagine you are sitting on a chair in your living room and are motionless relative to Chewbacca a billion light years away (a glight year? :p). What happened to Chewie a few minutes ago could be said to have happened a few minutes ago. However, if you get up and start walking across the room at a reasonable speed, then what Chewie did a few minutes ago in his past is thousands of years ago in your past.

    I don’t claim to understand that or how it works exactly, so even if I am completely wrong, the underlying point which you two brought up remains:
    Over huge distances even small inconsistencies in reference frames renders the concept of “simultaneous” events rather meaningless. As such, “10,000 earth years” doesn’t really mean anything when you aren’t talking about what happened very near to Earth.

    But enough of that: I found this post to be quite excellent. The calculated quantities of heavier elements thrown out by supernovae is so fascinated that before now I never even realized that I had been wondering it. Keep up the great work.

    Cheese Hater: a “klight year” is a typo, much like I hope your use of an apostrophe for pluralization was.

  4. PK

    By choosing the moment of the nova some 300+ years ago, you choose a specific convention for simultaneity. This is OK, and on the scale of the universe this might be the only meaningful way to talk about simultaneous events. However, on the scale considered here we can assume spacetime is flat, and we can thus use the more “conventional” convention ;-) in which the nova happened some 10k years ago with respect to the earth’s frame of reference.

    PS. I believe that simultaneously is the only word in the English language that uses all vowels (exactly once).

  5. Simple Guy

    “We cannot say anything about that event until the light reaches us, and in a real sense that event has not happened until the light reaches us.”

    Huh? I understand that perhaps we cannot interpret new “data” until we receive it. But, you want to say it hasn’t happened until we can perceive it? I can understand the event didn’t happen “to us” until we received the light. But, to say it didn’t happen earlier doesn’t make sense…unless I am not taking into account some theory of physics.

    Now, it would be interesting to have a new word (such as k-light) to indicate when light from an event significantly far away reached us. :-)

  6. Will M.

    Let’s see: if time is like a river, one can never step into the same river twice; or, step into a river in the same place twice; if a tree falls in the forest and nobody hears it, it didn’t fall, or – I’m getting totally bollixed. I always thought it was really neat to think that the starlight we see on Earth really was like looking into a time machine – like looking into the past – and if we could somehow devise a tool to “translate” that light we might actually be able to see the beginnings of the event which brought the light to us in the first place. I guess these ideas just demonstrate my total lack of knowledge re physics (and metaphysics, too)…

  7. Tim G

    Iron is the ultimate nuclear ash. It has a
    tightly bound nucleus
    and consequently a low energy state. I guess we’ll be seeing more iron in the future. Comparatively little lithium is produced in supernovae and we can see why from the binding energy chart.

  8. Navneeth

    Okay, let’s all forget about simultaneity, light cones and stuff for a moment. If no one observed the event from Earth, how were the astronomers able to estimate that its light reached Earth in the year 1680? – that’s pretty sharp as far as astronomy is concerned.

  9. The Galaxy Trio

    Nnnnnno… it happened 10,000 years ago.

    And yes, I grok the other arguments. They’re unecessry obfuscation when dealing with objects at non-relativistic velocities.

    It happened 10,000 years ago. Period. We can say that because we know when the light reached us, where it came from, and how far away the light source is. There’s some corrections to be made for relative motion of Earth and Cas A (another known or knowable figure), by 10 KYears is probably about right.

    The oldest “light” we can see if the microwave background radiation, and that dates, if I recall, to roughly 1/2 million years after the Big Bang. Yet we happily talk about what happened before that, all the way back to 10^-23 seconds after the Bang.

  10. Well, to be completely accurate about when it occured, we’d have to all for all possible frames of simultaneity. This then gives us sometime before 1,680 AD and after 18,320 BC. (Though the latter is probably inaccurate in a number of ways, taking into account the relative motion of the earth to that star and the inaccuracy in the estimate of 10,000 lightyears. So, to keep significant figures, let’s generalize it to 20,000 BC.)

  11. Evolving Squid

    PS. I believe that simultaneously is the only word in the English language that uses all vowels (exactly once).

    facetious (also uses each vowel in order)

    behaviour, sequoia

    uncomplimentary (uses each vowel once, including Y)

    There are many others :)

  12. Evolving Squid

    I suppose it’s really a Schrödinger’s Cat sort of thing. Once the light has reached us, we measure it and conclude that the light left its source 10,000-ish years ago, but until we measure it, the state is unknown and not definable. From that point of view it “happens” when the light arrives and we make relative judgments against our calendar.

    Because the state is not knowable until the light arrives, it seems fair to say it hasn’t happened, and when the state is known, we retcon it.

  13. Jethro

    How about “as observed from the earth’s frame of reference, the worldline of the earth intersected the future light cone of the explosion in the year 1680″

  14. Brian Lacki

    Going off what I remember from my relativity class a little while ago…

    We cannot say anything about that event until the light reaches us, and in a real sense that event has not happened until the light reaches us. Time flows like light, I sometimes say, meaning that the event itself happens when the light reaches us. So it is acceptable to say that the explosion actually happened in 1680.

    By that same logic, couldn’t we say that the Cas A supernova occurred at the same place as the Earth? A light cone is a null surface. That means that the distance of any path along the light cone is zero. If you think about it, this makes sense.: a photon traveling along the light cone will experience time dilation and length contraction. The time “experienced” by an observer riding the photon (its proper time) will be zero because of the time dilation. Similarly, the distance “experienced” by that observer (its proper distance) will be zero, because of the length contraction. In that sense, the supernova and the light reaching the Earth occur at the same place. Most people wouldn’t actually put it that way, though.

    Note that the zero distance along the light cone is not relative. In relativity, space and time are melded into spacetime. The geometry of spacetime is absolute. The distance between two points in spacetime is given by that geometry. It’s invariant under coordinate transformation — that is, whatever strange way we label points in spacetime, the distance between those points doesn’t change [1]. (In the sort of the same way, the distance between places on Earth does not depend on whether we’re using a Meractor projection or a cylindrical projection map of the Earth.) If the distance between two points in spacetime is zero, we say that they are lightlike separated [2]. If the distance between them is positive, we say that they are spacelike separated, and they have no way of knowing of each other; they are outside of each others’ light cones. If the distance between them is negative, then they are timelike separated, and they are within each others light cones [3].

    To label points uniquely in spacetime, we need four coordinates. At a given point, one of these will serve as a temporal coordinate, and three as a spatial coordinate (this is another property of spacetime — but which coordinate acts as time can change within your coordinate system depending on where you are [4]). No coordinate system is privileged – they all work well. That’s why time and space are relative. But spacetime, considered as a whole, is absolute [5].

    Consider, you say that the supernova occurred about 300 years ago. But how can you say how long ago the light reached Earth from now? How are you measuring the distance from Earth right now to the point where light reached Earth from Cas A? You could use, say, the proper time experienced by Earth in the intervening time. This would always give you a consistent answer, since that proper time is absolute. But why not use the proper time experienced by a hypothetical rocket traveling at 50% of the speed of light away from Earth to the Galactic Center? If you use the proper time experienced by the astronauts, you will still get a consistent answer. Basically, you need some way to name your light cones uniquely. You chose to name them with the proper time experienced by Earth, but that’s not the only way you could do it.

    Similarly, while the proper distance between the Cas A supernova and Earth in 1680 is zero, that does not mean that they actually were at the same point. Otherwise, Earth would’ve been might toasty. To have a useful coordinate system, you need to have a way of saying where on Earth’s light cone the supernova was. You say that the Cas A remnant is (and was) 10,000 light years away. Which coordinate system are you using? If you use the hypothetical astronaut’s coordinate system, you’ll get something different, and the astronaut’s coordinate system is just as good as yours. The point is, you need three more numbers to uniquely name a point on your light cone, for a grand total of four numbers. But again, you could’ve labeled it differently.

    Now, your coordinate system, as far as I can see it, is perfectly valid. When someone complains that Cas A was 10,000 years ago, not 300 years ago, that’s because they’re saying they’re using a different coordinate system than you are. When they think of a specific “moment” in time, think of a specific spacelike surface in spacetime. You carve up spacetime differently – your “moment” is a lightlike surface. You are perfectly in your right to do this. In fact, that sort of thing can happen naturally in general relativity [4]. But you can be no more right than they are — your coordinate system is just as relative as theirs. When you say the Cas A supernova happened in 1680 and 10,000 light years away, that’s just your label for a specific point in spacetime. You could’ve labeled it differently, even if you do use light cones as moments of time, by naming those light cones and the points on those light cones differently. You can’t claim to be more correct just because the coordinates are relative, since it’ll apply to you as well.

    So, it all comes down to simply a matter of convenience. Just because a coordinate system is valid, doesn’t mean it’s useful. For everyday life, both results will give pretty much the same results. There are some advantages, in that you don’t have to say things like “Supernova 1987A happened 1987 minus 180,000 years”, or “The Mars probe has now deployed its parachutes, and we’re getting the telemetry for that.” It can be a bit confusing, though — because other people carve up spacetime into different moments in a different way than you do. And someone could just as easily use the light cones for one of the spatial coordinates instead of one of the temporal coordinates. Usually, people just say which coordinate system they are using. Timelines for space probe events, for example, often account for light delay, and just give the time when we should recieve the probe’s radio signals from an event.

    Um, closer to topic, are we sure the Cas A supernova was caused by a massive star? I ask that because I vaguely remember hearing it was suspected to be a Type Ia supernova, in which case it would’ve been a white dwarf (or two?) exploding. The basic point would still be the same — type Ia supernovae produce a lot of iron, especially since the entire white dwarf undergoes fusion, and all of it is blown away. Or have they pretty much ruled that out?

    [1] That’s basically the solution to the Twin Paradox, if someone’s wondering. In general relativity, the astronaut’s coordinate system is just as valid as Earth’s. But even so, when you integrate the distance traveled by the rocket, it will always come out as shorter than the Earth’s, even in the astronaut’s coordinate system.

    [2] This may be a bit confusing: in relativity, just because two points are separated by a proper distance of zero, does not mean they are the same point. If readers are interested, in flat spacetime, the proper distance between two points is found by:

    ds^2 = -(c dt)^2 + (dx)^2 + (dy)^2 + (dz)^2

    (If someone doesn’t know what the “d”s are, just ignore them for now. They’re sort of like deltas.) This is basically the Pythagorean Theorem for spacetime. It’s because of the minus sign that you can get the zero distances, if (c dt) is equal to the distance in space. It’s also the reason we get all sorts of weird special relativistic effects.

    [3] Yes, that’s even true when you have things like wormholes or Alcubierre warp drives. They provide a shortcut between points in spacetime that would look far away in flat spacetime. When you fall through a wormhole, for example, you are still within your past self’s light cone — it’s just that the light cone as a whole has a weird shape. Of course, wormholes and warp drives have other problems.

    [4] The event horizon of a black hole is lightlike. Think about it. If you’re falling into a black hole, and you emit a photon upwards as you cross the event horizon, that photon can never escape. But it won’t fall in, either. It’ll stay trapped at the event horizon forever.

    In radial coordinate systems, where the event horizon is defined as being at radius r = R, then r = R is a lightlike surface. For the Schwarzchild coordinates, surfaces of constant r are spacelike inside the event horizon. That is, the distance r from the singularity acts like a spatial coordinate outside of the horizon. Inside the horizon, it acts like a time coordinate.

    That’s also why you can’t escape from a black hole once you pass the horizon. The singularity, at r = 0, acts more like (what most people would call) a moment in time than a point in space. Once you are in the horizon, the singularity covers your entire future light cone. Assuming general relativity holds all the way in.

    (By the BA’s way of defining a moment in time, the event horizon would be a specific time instant, not the singularity, though.)

    [5] That’s also why you don’t really need to talk about time “flowing” in relativity. Time is on the same footing as space, because they are both integral part of spacetime. So, one could just as easily argue that space “flows”. Similarly, how would you say which moment it is right now? The very way you define “now” depends on which way you carve up spacetime.

  15. PK

    Evolving Squid, you’re right about the words containing all vowels, but the conventionality of simultaneity is nothing like Schroedinger’s cat! In classical (as opposed to quantum) relativity theory, events are perfectly well-defined with respect to any reference frame, and have a definite temporal order as well. Schroedinger’s cat, on the other hand, is neither dead nor alive before measuring whether it is dead or alive. The weird bits of relativity do not equal the weird bits of quantum mechanics!

  16. navneeth: the nebula is seen to expand, and the rate can be measured. Knowing how big it is can then tell you hold it is. Example: a car moving at 50 km/hr is seen to be 100 km from its origin. Therefore it’s been gone 2 hours.

    That assumes constant speed and all that, but that’s the principle here.

  17. xav666

    I disagree with Phil’s side note. Just because you are not aware of something doesnt mean it doesnt exist. I often wonder how many stars in the universe we can not see that exist (just being too faint to be seen from our telescopes.)

  18. Navneeth

    the nebula is seen to expand, and the rate can be measured. Knowing how big it is can then tell you hold it is. Example: a car moving at 50 km/hr is seen to be 100 km from its origin. Therefore it’s been gone 2 hours.

    That assumes constant speed and all that, but that’s the principle here.

    That’s good enough for me, for the moment. Thanks, BA. :)

  19. MaDeR

    Damn, you are all discuss about mundane things. But I think that everyone should at least once thought about it: WE ARE ALL CHILDREN OF STARS. In real, literal sense.

  20. icemith

    Love that post by Brian Lacki,(August 29, 2006, 3:40pm). It’s a fairly comprehensive description of space/time etc., I just wish I could get my head around even half of it. I will just have to take his word for it.

    But what I really don’t understand is that there does not appear to be a word to explain the concept of the actual happening of two, or more, even millions of events, similtaneously. We *know* they happen – it is a dynamic universe – so they must all have an effect on each other, however slight or massive.

    I sometimes wonder that there may be an effect on us at that actual moment, possibly insignificant, but there nevertheless. That the visual display of it, and/or the gravitational and/or ionic effect, or even the blast wavefront as it overwhelms our little Third Rock, or whatever, may not reach us for 8 minutes/years/millenium/lightyears… is the tangible evidence of that event. I think I can safely say that at this moment somewhere in our universe a mighty event is happening, but it won’t annoy us until we see it, and then it will be too late. It is a bit like the disclaimer on the ads for the lottery promotion – “the Prize may have already have been won before your ticket is bought”. (Or the thought that with FOI provisions, what our politicians are doing now, will be in the papers in 30 years, showing them in a different light.)

    It is generally agreed that nothing travels faster than Light, not even Gravity, and I know this is 101 stuff, but what about in-tangibles? Thought, Perception and even Knowledge are in that grab-bag. (Sorry for the capitalisation of those words – it’s a literary thing, nothing to do with the supernatural!). Or is there something else?


  21. Kessa

    It looks like Mr. Burns!

  22. Gary Ansorge

    Gabble, gabble, gabble. Now here’s my two bits,,,If you are moving at the speed of light(remember, this is a thought experiment) you would travel throughout the entire universe, experience everything there is, in zero personal time. Unfortunately, you couldn’t remember ANY of those experiences, because time is the separator of cause and effect. Time is realitys way of keeping everything from happening at once,,,

    Wow, time, what a concept,,,

    GAry 7

  23. gopher65

    I was thinking that it looks kinda like a Zerg Hydrolisk.

  24. PeterH

    > PS. I believe that simultaneously is the only word in the English language that uses all vowels (exactly once).

    Okay, so then is the second “u” silent?


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